[DOC] Missing references

[josephsalmon]

I have realized that most solvers do not have a clear reference.
This includes:

• ADMM.py (may be this: https://stanford.edu/~boyd/papers/admm_distr_stats.html ?)
• ChambollePockPDsplit.py (ref clear from name)
• CondatVu.py (ref clear from name)
• DPGD.py (not sure which ref to use).
• FP.py (no idea what would be the source)
• ISTA.py (ref : Beck and Teboulle 2009?)
• PGD.py (ref: prox book by Boyd? : https://web.stanford.edu/~boyd/papers/pdf/prox_algs.pdf ?)

May be @svaiter or @tomMoral you have in mind the missing ones?